The present invention relates to a cold-cathode device for field-emitting electrons, and a vacuum micro device, such as an image display device, employing the cold-cathode device.
Recently, field emission cold-cathode devices using semiconductor processing technologies are being actively developed. As one representative example, a device described by C. A. Spindt et al. in Journal of Applied Physics, Vol. 47, 5248 (1976) is known. This field emission cold-cathode device is manufactured by forming an SiO2 layer and a gate electrode layer on an Si single-crystal substrate, forming therein a hole having a diameter of about 1.5 xcexcm, and forming a conical emitter for performing field emission in this hole by vapor deposition while rotating the Si single-crystal substrate.
However, the above manufacturing method and the field emission cold-cathode device manufactured by the method have the following problems.
First, a rotational deposition method is performed such that the diameter of the pinhole formed in the gate electrode gradually decreases, thereby forming the shape of the emitter in the hole. For this reason, the height of the emitter and the shape of the tip of the emitter vary, and this degrades the uniformity of field emission. Additionally, the reproducibility of the shape and the yield are low. This greatly increases the production cost in manufacturing a large number of field emission cold-cathode devices having uniform characteristics on a single substrate.
Further, since the tip of the emitter necessary to improve the field emission efficiency lacks sharpness, the driving voltage is increased. This poses problems such as a reduction in the field emission efficiency and an increase in the consumption power. When a high driving voltage is used, the shape of the emitter tip readily changes under the influence of a residual gas ionized by this voltage. This also raises problems in terms of reliability and service life.
Furthermore, where a high driving voltage is used, an arc discharge, which degrades and/or destroys emitters, is caused, thereby making the electron device lose its function. For this reason, the electron device is generally set at a vacuum pressure of 10xe2x88x925 Torr or less, and preferably 10xe2x88x926 Torr or less, at which arc discharges are prevented from being caused, nevertheless an arc discharge still occasionally occurs.
The present invention has been made to solve the above problems, and one of its objects is to provide a field emission cold-cathode device having uniform field emission characteristics, capable of being driven with a low voltage, and also having a high field emission efficiency, and a vacuum micro device, such as an image display device, employing the cold-cathode device.
According to a first aspect of the present invention, there is provided a field emission cold-cathode device comprising:
a support member; and
an emitter formed on the support member to emit electrons, the emitter comprising at a surface an electron-emission layer including a first part consisting essentially of a first conductive material having a work function of 4.0 eV or less, and a second part arranged in contact with the first part and consisting essentially of a second conductive material having a negative electron affinity, one of the first and second parts comprising granular bodies or linear bodies each having a diameter of 100 nm or less.
According to a second aspect of the present invention, there is provided a vacuum micro device comprising:
a support member;
an emitter formed on the support member to emit electrons, the emitter comprising at a surface an electron-emission layer including a first part consisting essentially of a first conductive material having a work function of 4.0 eV or less, and a second part arranged in contact with the first part and consisting essentially of a second conductive material having a negative electron affinity, one of the first and second parts comprising granular bodies or linear bodies each having a diameter of 100 nm or less;
a surrounding member for forming, together with the support member, a vacuum discharge space surrounding the emitter; and
an extracting electrode arranged to be spaced apart from the emitter, the emitter emitting electrons due to a potential difference between the emitter and the extracting electrode.
According to a third aspect of the present invention, there is provided an image display device comprising:
a support member;
an emitter formed on the support member to emit electrons, the emitter comprising at a surface an electron-emission layer including a first part consisting essentially of a first conductive material having a work function of 4.0 eV or less, and a second part arranged in contact with the first part and consisting essentially of a second conductive material having a negative electron affinity, one of the first and second parts comprising granular bodies or linear bodies each having a diameter of 100 nm or less;
a surrounding member for forming, together with the support member, a vacuum discharge space surrounding the emitter;
an extracting electrode arranged to be spaced apart from the emitter, the emitter emitting electrons due to a potential difference between the emitter and the extracting electrode; and
a display portion for displaying an image in accordance with excitation by electrons emitted from the emitter, the display portion being turned on and off under a control of the potential difference between the emitter and the extracting electrode, on which emission of electrons from the emitter depends.
In the present invention, an emitter has an electron-emission layer including a first part consisting essentially of a first conductive material having a work function of 4.0 eV or less (low-work-function material), and a second part arranged in contact with the first part and consisting essentially of a second conductive material having a negative electron affinity (NEA material). At least one of the first and second parts comprises granular bodies or linear bodies each having a diameter of 100 nm or less, and preferably 30 nm or less, but not less than 1 nm.
The tip of each linear body is also set to have a radius of curvature of 50 nm or less, and preferably 15 nm or less. With the combination of these features in materials and shapes, electrons are easily emitted from the emitters by field emission, so that the device can be driven with a low voltage, and have field emission characteristics improved to be uniform and stable.
Especially, where the second part of a NEA material positioned on the upper side, electrons are supplied from the cathode electrode through the first part of a low-work-function material, and emitted from the second part of a NEA material. In this case, the driving voltage is further reduced while improving stability and uniformity of the emitted current, because the Schottky barrier height in the electron-emission layer is lowered, and NEA materials present excellent field emission characteristics.
Further, since at least one of the first and second parts comprises granular bodies or linear bodies, the emitter need not to be entirely formed of granular bodies or linear bodies. If the entirety of the emitter were to be formed of granular bodies or linear bodies, these bodies would be hardly stably adhered to each other. Where only the surface portion of the emitter is formed of granular bodies or linear bodies, these bodies can be adhered to each other more easily.
In a flat-type emitter, the conductive support layer also works as an adhesion layer for fixing the granular bodies or linear bodies in the electron-emission layer, and thus the bodies can be easily fixed. Further, the conductive support layer allows a voltage to be uniformly applied, thereby preventing an abnormal discharge.
Where a ballast resistor layer made of a resistive material is included in the conductive support layer, the device is provided with a current restriction effect, which can reduce the degree of damage, degradation, and current fluctuation due to overcurrent or current fluctuation, so that a more stable emitted current is obtained.
The above described advantages in a flat-type emitter are further enhanced by a convex emitter preferably with a gate electrode. In this case, it is possible to fix an electron emission point and to control the device more easily so as to further improve uniformity and stability of the emitted current, thereby preferably applying the device to a flat-type image display device.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.